Ultrafast dynamics of exciton fission in polycrystalline pentacene

We use ultrafast transient absorption spectroscopy with sub-20 fs time resolution and broad spectral coverage to directly probe the process of exciton fission in polycrystalline thin films of pentacene. We observe that the overwhelming majority of initially photogenerated singlet excitons evolve into triplet excitons on an ～80 fs time scale independent of the excitation wavelength. This implies that exciton fission occurs at a rate comparable to phonon-mediated exciton localization processes and may proceed directly from the initial, delocalized, state. The singlet population is identified due to the brief presence of stimulated emission, which is emitted at wavelengths which vary with the photon energy of the excitation pulse, a violation of Kasha's Rule that confirms that the lowest-lying singlet state is extremely short-lived. This direct demonstration that triplet generation is both rapid and efficient establishes multiple exciton generation by exciton fission as an attractive route to increased efficiency in organic solar cells.     

Intermolecular intersystem crossing in single-molecule spectroscopy: Terrylene in anthracene crystal

We present a spectroscopic study of terrylene in anthracene crystals at the ensemble and single-molecule levels. In this matrix, single-molecule fluorescence is reduced by three orders of magnitude. Correlation measurements allow us to identify a new relaxation channel, matrix-enhanced intersystem crossing. This process starts with a singlet-to-triplet energy transfer from guest to host, after which the triplet exciton is transferred back to the guest. The intermolecular intersystem crossing is expected whenever the lowest triplet state of the host is located between the lowest singlet S(1) and lowest triplet T(1) excited states of the guest. It must be considered when searching for new host-guest systems for single-molecule spectroscopy.     

Hole-induced quenching of triplet and singlet excitons in conjugated polymers

Quantitative information on the mechanisms and rates of hole (radical cation)-induced quenching of triplet and singlet excitons in the conjugated polymer poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] has been acquired by a new technique, fluorescence-voltage time-resolved single molecule spectroscopy (FV-TR-SMS). FV-TR-SMS measures the fluorescence intensity of a single conjugated polymer molecule that is embedded in a capacitor-like device while simultaneously modulating the bias on the device and the irradiation intensity. The results demonstrate that triplet excitons are efficiently quenched by holes in conjugated polymers for hole densities >10(16) charges/cm(3), while singlet excitons are quenched with a much lower efficiency. Detailed kinetic analysis shows that the greater efficiency for quenching of triplets by holes (compared to that for singlets) is due to a >10(6) times longer exciton lifetime for triplets. In fact, the results suggest that while singlet quenching is less efficient due to a much shorter singlet lifetime, the rate constant for the quenching of singlets by holes actually exceeds that for triplets by several orders of magnitude.     

Singlet energy migration along an alternating block copolymer of oligothiophene and oligosilylene in solution

The singlet excited-state properties of the block copolymers of oligothiophene and oligosilylene in solution were investigated with several fast spectroscopic methods. Time-resolved fluorescence measurements at room temperature and in a glassy matrix revealed that the singlet excited states of the block copolymers are deactivated accompanying structural changes of the polymer. It became clear from the transient absorption spectroscopy that the absorption peak of the singlet excited state shifted to the longer wavelength side compared to that of the corresponding oligothiophenes because of the sigma-pi conjugation of the oligothiophene and oligosilylene. The intersystem crossing process generating the triplet excited state was also revealed by the transient absorption spectroscopy. Energy migration along the polymer chain was revealed by the fluorescence anisotropy measurements. The time constant for the energy migration became faster as the size of the oligothiophene in the polymer repeating unit became shorter. From comparison with the F?rster theory, the energy migration process was attributed to an incoherent hopping mechanism.     

Role of hydrogen bonding in the photophysical properties of isomeric tetrapyridylporphyrins in aprotic solvent

Extensive photophysical properties of isomeric tetra-2-pyridylporphyrin (TpyP(2)), tetra-3-pyridylporphyrin (TpyP(3)), and tetra-4-pyridylporphyrin (TpyP(4)) have been studied in the presence of a series of phenols of increasing hydrogen bonding power in dichloromethane solution by employing UV/vis spectroscopy; steady-state, time-resolved fluorescence spectroscopy; and transient absorption spectroscopic techniques. The change of absorption spectra of all three porphyrins as a function of different phenol concentrations established the preference of hydrogen bonded complex formation to the peripheral pyridyl nitrogen rather than the pyrrole nitrogen of the porphyrin macrocycle. The fluorescence behaviors of the porphyrins which were observed upon addition of different phenols point to a marked dependence on the nature of the added phenols. Phenols with an electron withdrawing group do not quench the fluorescence of porphyrins, whereas phenols with an electron donating group quench the singlet porphyrin both in static and dynamic pathways. A remarkable difference in quenching behaviors of singlet excited porphyrin by 4-methylphenol (4-MePhOH) and 4-MeOPhOH/4-EtOPhOH (4-EtOPhOH = 4-ethoxyphenol) are observed. The quenching of singlet excited porphyrins by 4-MePhOH is attributed to be purely static in nature, and the H-bond provides a strong nonradiative channel to singlet excited porphyrins. However, the quenching of singlet excited porphyrins by 4-MeOPhOH/4-EtOPhOH is mostly dynamic, and it is ascribed to be the reductive quenching of single excited porphyrins. Picosecond transient absorption study with TpyP(2) and 4-MeOPhOH provides the evidence of porphyrin radical anion and phenol radical cation of equal lifetime, which indicates the fact that electron transfer occurs from phenol to singlet excited porphyrin. The temperature effect on dynamic quenching by 4-MeOPhOH/4-EtOPhOH and kinetic deuterium isotope effect established the reaction to be a photoinduced concerted proton coupled electron transfer.     

Enhanced photogeneration of triplet excitons in an oligothiophene-fullerene blend.

Photoinduced and transient absorption spectroscopy is used to study triplet exciton dynamics in thin films of a new thiophene-based oligomer (DCV3T) and blends of DCV3T and fullerene C60. We find enhanced DCV3T triplet exciton generation in the blend layer, which is explained as an excitonic ping-pong effect: singlet energy transfer from DCV3T to C60, followed by immediate intersystem crossing to C60, and triplet exciton back-transfer. Estimations of the rate constants involved show that the ping-pong effect has an overall efficiency close to unity. The singlet-singlet energy transfer from DCV3T to C60 is demonstrated by efficient quenching of DCV3T luminescence in the blend, leading to sensitized emission of C60. We discuss a promising new concept of solar cells with an enlarged active-layer thickness based on potentially long-ranged triplet exciton diffusion in combination with efficient intersystem crossing.     

Exciton migration with end detectors: calculation of time-dependent luminescence

We present explicit plots of the time dependence of the detector luminescence intensity in surface quenching observations. They are obtained with the aid of numerical inversion methods applied to exact consequences of a master-equation theory of exciton migration. The existing disparity in reported values of the singlet diffusion constant in aromatic hydro-carbon crystals is one of the main issues addressed. It is shown how the theory can be used to extract the exciton diffusion constant without prior knowledge of the (detector) trapping rates, by combining information obtained from steady-state (yield) and time-resolved experiments.     

Single molecule modulation spectroscopy of conjugated polymers

This paper describes a new single molecule spectroscopy approach for the investigation of triplet-triplet and singlet-triplet interactions in conjugated polymers. The technique involves the irradiation of isolated single, mulitchromophoric, conjugated polymer molecules by a repetitive sequence of variable-intensity microsecond time scale excitation pulses. The fluorescence intensity is synchronously time-averaged for thousands of cycles of the pulse sequence to yield a high signal-to-noise fluorescence transient on the microsecond time scale. The transient can be analyzed with kinetic models to obtain quantitative information about the kinetics of triplet-triplet exciton annihilation and the quenching of singlet excitons by triplet excitons in conjugated polymers.     

Size-dependent exciton chirality in (R)-(+)-1,1'-bi-2-naphthol dimethyl ether nanoparticles

Organic nanoparticles from a chiral auxiliary, (R)-(+)-1,1'-bi-2-naphthol dimethyl ether (BNDE), with a range of particle sizes from 25 to 100 nm were fabricated through the reprecipitation method. It is found that BNDE nanoparticles exhibit positive exciton chirality in 200-260 nm region in circular dichroism (CD) spectra, which are completely opposite to CD spectra of the dilute solution. The exciton chirality of the particles displays size-dependent behavior; that is, the exciton chirality peaks evolve to the low-energy side with increase in particles size. CD spectra accompanied with UV, fluorescence spectra, lifetime measurements of the excited states, and quantum mechanical calculations reveal that the chirality inversion results from intermolecular exciton coupling between two adjacent BNDE molecules in the nanoparticles, and the bathochromic shift of the peaks is attributed to the increased intermolecular interaction with increasing particle size.     

Determination of the electron transfer parameters of a covalently linked porphyrin-quinone with mesogenic substituents - optical spectroscopic studies in solution

The photoinduced electron transfer (PET) of a covalently linked porphyrin-quinone with mesogenic substituents was studied using visible and near-IR (NIR) spectroscopy. Mesogenic substituents were introduced at the porphyrin moiety in order to mimic the anisotropic membrane properties of the native reaction centre of photosynthesis. Photophysical characterization of this system in homogeneous solution is a prerequisite for a better understanding of the effects occurring in anisotropic medium. For this reason, we studied the fluorescence and phosphorescence quenching and lifetime of the charge-separated state. Time-resolved fluorescence measurements indicated an effective singlet PET. The complete set of PET parameters was calculated using Marcus theory of non-adiabatic electron transfer (ET). Steady state measurement of singlet oxygen luminescence, which allows indirect access to phosphorescence quenching, indicated that no triplet PET was involved in the decay processes. Using transient absorption spectroscopy, the lifetime of the charge-separated state was found to be 1.9 ns.     

Photophysical characterization of light-emitting poly(indenofluorene)s.

Time-resolved photoluminescence spectroscopy experiments of three poly(2,8-indenofluorene) derivatives bearing different pendant groups are presented. A comparison of the photophysical properties of dilute solutions and thin films provides information on the chemical purity of the materials. The photophysical properties of poly(2,8-indenofluorene)s are correlated with the morphological characteristics of their corresponding films. Wide-angle X-ray scattering experiments reveal the order in these materials at the molecular level. The spectroscopic results confirm the positive impact of a new synthetic approach on the spectral purity of the poly(indenofluorene)s. It is concluded that complete side-chain substitution of the bridgehead carbon atoms C-6 and C-12 in the indenofluorene unit, prior to indenofluorene ring formation, reduces the probability of keto formation. Due to the intrinsic chemical purity of the arylated derivative, identification of a long-delayed spectral feature, other than the known keto band, is possible in the case of thin films. Controlled doping experiments on the arylated derivative with trace amounts of an indenofluorene-monoketone provide quantitative information on the rates of two major photophysical processes, namely, singlet photoluminescence emission and singlet photoluminescence quenching. These results allow the determination of the minimum keto concentration that can affect the intrinsic photophysical properties of this polymer. The data suggest that photoluminescence quenching operates in the doped films according to the Stern-Volmer formalism.     

Structure and photophysical properties of porphyrin-modified metal nanoclusters with different chain lengths

Three-dimensional porphyrin-monolayer-protected gold clusters with different chain lengths (MPCs) have been prepared to examine the structure and photophysical properties, in comparison with self-assembled monolayers (SAMs) of the porphyrins on a flat gold surface. The three-dimensional porphyrin MPCs exhibit electrochemical and photophysical properties that are much closer to those of a porphyrin reference compound in solution than those of two-dimensional porphyrin SAMs on the flat gold surface. The three-dimensional architectures of porphyrin MPCs with large surface area have improved the light-harvesting efficiency relative to the corresponding porphyrin SAM on the two-dimensional flat gold surface. Time-resolved single photon counting fluorescence and transient absorption spectroscopic studies have demonstrated that undesirable quenching of the porphyrin excited singlet state via energy transfer to the gold surface of the three-dimensional MPCs is much suppressed, as compared to the quenching of the porphyrin SAMs on the two-dimensional flat gold surface. Both the quenching rate constants of the porphyrin excited singlet state by the surfaces of bulk gold and gold nanoclusters reveal weak chain length dependence of the energy transfer quenching.     

单重态激子裂分的研究进展

单重态激子裂分指的是在有机分子中一个单重态激子与相邻的基态发色团相互作用形成两个三重态激子的过程。利用这种多激子效应制成的光伏器件有望突破肖克利-奎瑟限制,使光电转换的理论效率由30%提高到44.4%。近年来各国科学家在裂分材料的设计合成和器件化应用方面取得了一定的进展,但是对于激子裂分物理本质的认知仍然存在争议和分歧。本文较为系统地介绍了激子裂分材料的最新进展和本研究组的相关工作。简要回顾了激子裂分的发展历史,从概念、裂分的发生条件和作用机制三方面介绍了激子裂分过程,综述了具有分子间和分子内裂分性质的材料的最新研究成果。在系统归纳激子裂分研究现状的基础上对单重态激子裂分的发展趋势和应用探索指出了可能的方向。     

Singlet oxygen and polymer photooxidations. I. Sensitizers,quenchers, and reactants

A study designed to ascertain the role of singlet molecular oxygen in the photodegradation of plastics established that most classes of dye chromophores are sensitizers in polymer films, absorbing light and transferring the absorbed energy to ambient triplet ground state molecular oxygen, generating metastable reactive singlet molecular oxygen. Unsaturated polymers containing polybutadiene, polyisoprene, etc. are highly reactive to singlet oxygen produced through photosensitization, generating hydroxylic and carbonyl derivatives and losing their rubbery properties as consequences of such reactions. Many types of transition metal chelates are singlet oxygen quenchers. The relationships of the structures and spectroscopic properties of these chelates to their efficiency in quenching singlet oxygen are examined and discussed.     

Experimental and theoretical study of temperature dependent exciton delocalization and relaxation in anthracene thin films

The spectroscopy of solid anthracene is examined both experimentally and theoretically. To avoid experimental complications such as self-absorption and polariton effects, ultrathin polycrystalline films deposited on transparent substrates are studied. To separate the contributions from different emitting species, the emission is resolved in both time and wavelength. The spectroscopic data are interpreted in terms of a three-state kinetic model, where two excited states, a high energy state 1 and a low energy state 2, both contribute to the luminescence and are kinetically coupled. Using this model, we analyze the spectral lineshape, relative quantum yield, and relaxation rates as a function of temperature. For state 1, we find that the ratio of the 0-0 vibronic peak to the 0-1 peak is enhanced by roughly a factor of 3.5 at low temperature, while the quantum yield and decay rates also increase by a similar factor. These observations are explained using a theoretical model previously developed for herringbone polyacene crystals. The early-time emission lineshape is consistent with that expected for a linear aggregate corresponding to an edge-dislocation defect. The results of experiment and theory are quantitatively compared at different temperatures in order to estimate that the singlet exciton in our polycrystalline films is delocalized over about ten molecules. Within these domains, the exciton's coherence length steadily increases as the temperature drops, until it reaches the limits of the domain, whereupon it saturates and remains constant as the temperature is lowered further. While the theoretical modeling correctly reproduces the temperature dependence of the fluorescence spectral lineshape, the decay of the singlet exciton appears to be determined by a trapping process that becomes more rapid as the temperature is lowered. This more rapid decay is consistent with accelerated trapping due to increased delocalization of the exciton at lower temperatures. These observations suggest that exciton coherence can play an important role in both radiative and nonradiative decay channels in these materials. Our results show that the spectroscopy of polyacene solids can be analyzed in a self-consistent fashion to obtain information about electronic delocalization and domain sizes.     

Method for accurate experimental determination of singlet and triplet exciton diffusion between thermally activated delayed fluorescence molecules

Understanding triplet exciton diffusion between organic thermally activated delayed fluorescence (TADF) molecules is a challenge due to the unique cycling between singlet and triplet states in these molecules. Although prompt emission quenching allows the singlet exciton diffusion properties to be determined, analogous analysis of the delayed emission quenching does not yield accurate estimations of the triplet diffusion length (because the diffusion of singlet excitons regenerated after reverse-intersystem crossing needs to be accounted for). Herein, we demonstrate how singlet and triplet diffusion lengths can be accurately determined from accessible experimental data, namely the integral prompt and delayed fluorescence. In the benchmark materials 4CzIPN and 4TCzBN, we show that the singlet diffusion lengths are (9.1 ± 0.2) and (12.8 ± 0.3) nm, whereas the triplet diffusion lengths are negligible, and certainly less than 1.0 and 1.2 nm, respectively. Theory confirms that the lack of overlap between the shielded lowest unoccupied molecular orbitals (LUMOs) hinders triplet motion between TADF chromophores in such molecular architectures. Although this cause for the suppression of triplet motion does not occur in molecular architectures that rely on electron resonance effects (e.g. DiKTa), we find that triplet diffusion is still negligible when such molecules are dispersed in a matrix material at a concentration sufficiently low to suppress aggregation. The novel and accurate method of understanding triplet diffusion in TADF molecules will allow accurate physical modeling of OLED emitter layers (especially those based on TADF donors and fluorescent acceptors).

A method for measuring triplet diffusion between TADF molecules is presented, and implications of limited triplet diffusion for OLEDs discussed.     

One-dimensional exciton diffusion in perylene bisimide aggregates

The dynamics and mobility of excitons in J-aggregates of perylene bisimides are investigated by transient absorption spectroscopy with a time resolution of 50 fs. The transient spectra are compatible with an exciton delocalization length of two monomers and indicate that vibrational and configurational relaxation processes are not relevant for the spectroscopic properties of the aggregates. Increasing the pump pulse energy and in that way the initial exciton density results in an accelerated signal decay and pronounced exciton-exciton annihilation dynamics. Modeling the data by assuming a diffusive exciton motion reveals that the excitons cannot migrate freely in all three directions of space but their mobility is restricted to one dimension. The observed anisotropy supports this picture and points against direct Fo?rster-transfer-mediated annihilation between the excitons. A diffusion constant of 1.29 nm(2)/ps is deduced from the fitting procedure that corresponds to a maximal exciton diffusion length of 96 nm for the measured exciton lifetime of 3.6 ns. The findings indicate that J-aggregates of perylene bisimides are promising building blocks to facilitate directed energy transport in optoelectronic organic devices or artificial light-harvesting systems.     

Spectroscopy and photophysics of self-organized zinc porphyrin nanolayers. 2. Transport properties of singlet excitation

Exciton diffusion has been studied in 5-25-nm-thick films of zinc tetra-(p-octylphenyl)-porphyrin (ZnTOPP) spin-coated onto quartz slides by intentional doping with quenchers using steady-state as well as time-resolved fluorescence spectroscopy. The fluorescence spectra of the films are very similar to those of solutions, indicating emission from localized exciton states. From the dependence of the fluorescence quenching on the quencher concentration and fluorescence lifetime measurements, the exciton diffusion can be concluded to be quasi-one-dimensional with an exciton diffusion length of 9 +/- 3 nm and an intrastack energy-transfer rate constant of 10(11)-10(12) s(-1). From fluorescence anisotropy decay measurements, we conclude that neighboring stacks aggregate in a herringbone structure, forming ordered domains that are randomly oriented in the substrate plane. These measurements indicate an interstack energy-transfer rate constant of (7 +/- 2) x 10(10) s(-1).     

Ensemble and single-molecule fluorescence spectroscopy of a calcium-ion indicator dye

The spectroscopic properties of Calcium Green 2 (CG-2), a dual-fluorophore Ca(2+) indicator dye, were characterized by a combination of steady state and time-resolved ensemble spectroscopic measurements, molecular mechanics calculations and single-molecule fluorescence spectroscopy. It was found that in Ca(2+) free solutions, CG-2 exists primarily as a highly quenched intramolecular dimer, but when bound to Ca(2+), the molecule adopts an extended, fluorescent conformation. The difference in emission properties of these two CG-2 conformations is explained in terms of simple exciton theory. Through single-molecule fluorescence measurements, we have shown that the bulk increase in ensemble fluorescence intensity correlates with a simple statistical increase in the number of fluorescent molecules in solution. In addition, we have also observed that the majority of CG-2 molecules photobleach in a single step, despite the molecule possessing two distinct fluorophores. A small fraction of molecules photobleach in multiple steps or show a series of transitions between emissive and nonemissive fluorescent states ("blinking"). We rationalize these photophysical phenomena using a simple model based on dipole-dipole F?rster coupling between fluorophores in conjunction with irreversible photodamage to one of the constituent chromophores.     

Chemically induced dynamic electron polarization generated through the interaction between singlet molecular oxygen and nitroxide radicals

An absorptive chemically induced dynamic electron polarization (CIDEP) was generated by the quenching of singlet oxygen by nitroxide radicals (TEMPO derivatives). The spin polarization decay time of the nitroxide (measured by time-resolved EPR) correlates with the lifetime of singlet oxygen (measured by singlet oxygen phosphorescence spectroscopy). In addition, a deuterium isotope effect on the spin polarization decay time was observed, a signature of singlet oxygen involvement. With use of isotope labeled nitroxides (15N, 14N), the relative spin polarization efficiencies of TEMPO, 4-oxo-TEMPO, and 4-hydroxy-TEMPO by singlet oxygen were determined. The relative spin polarization efficiencies (per quenching event) decrease in the order 4-hydroxy-TEMPO > TEMPO > 4-oxo-TEMPO, whereas an opposite trend was observed for the total quenching rate constants of singlet oxygen by the nitroxides where the order is 4-hydroxy-TEMPO < TEMPO < 4-oxo-TEMPO.